With the globally growing trend away from nuclear power generation, it is expected that there will be a further growing demand for fossil energy in the future. Accordingly, it is assumed that there will be a growing demand for high-strength linepipes having a large diameter and a thick wall to increase transportation efficiency of natural gases and oils. To date, UOE steel pipes manufactured from thick steel plates have been mainly used as linepipes for high-pressure operation. Nowadays, however, since there is a strong demand to decrease the material costs of steel pipes, for example, to decrease the construction costs of pipelines and due to insufficient supply capacity of UOE steel pipes, there is a trend toward using electric resistance welded steel pipes and spiral steel pipes manufactured from hot-rolled steel sheets with higher productivity and lower cost than UOE steel pipes.
Pipelines are constructed in cold areas having, for example, large natural gas reserves in many cases. Therefore, steel sheets as a material for linepipes are required to have excellent low-temperature toughness as well as high strength. In addition, linepipes laid over a long distance tend to be affected by crustal movement. To prevent pipes from bursting due to pressure fluctuations therein when a pipeline fractures and the leakage of the transported gas occurs by some chance due to forced deformation caused by crustal movement, steel pipes are required to have deformability in the circumferential direction thereof, that is, a stably low yield ratio.
In such a situation, various techniques regarding a hot-rolled steel sheet as a material for a linepipe have been proposed. For example, Japanese Unexamined Patent Application Publication No. 63-227715 proposes a technique of manufacturing a hot-rolled steel sheet including heating a steel slab having a chemical composition containing C: 0.03 wt % to 0.12 wt %, Si: 0.50 wt % or less, Mn: 1.70 wt % or less, P: 0.025 wt % or less, S: 0.025 wt % or less, Al: 0.070 wt % or less, and at least one of Nb: 0.01 wt % to 0.05 wt %, V: 0.01 wt % to 0.02 wt %, and Ti: 0.01 wt % to 0.20 wt % to a temperature of 1180° C. to 1300° C., then performing hot rolling with a rough rolling finishing temperature of 950° C. to 1050° C. and a finish rolling temperature of 760° C. to 800° C., performing cooling at a cooling rate of 5° C./s to 20° C./s, starting air cooling at a temperature higher than 670° C., continuing air cooling for 5 seconds to 20 seconds, then performing cooling at a cooling rate of 20° C./s or more, and performing coiling at a temperature of 500° C. or lower. In addition, Japanese Unexamined Patent Application Publication No. 63-227715 states that, by using the manufacturing method described above, it is possible to manufacture a hot-rolled steel sheet having a tensile strength of 60 kg/mm2 or more (590 MPa or more), a low yield ratio of 85% or less, and a low-temperature toughness corresponding to a ductile-brittle transition temperature of −60° C. or lower.
In addition, Japanese Unexamined Patent Application Publication No. 2006-299413 proposes a technique including hot-rolling a slab having a chemical composition containing, by mass %, C: 0.01% to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01% to 0.10%, Nb: 0.005% to 0.10%, and one, two, or more of Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, in which Mneq (Mneq (%)=Mn+0.26Si+3.5P+1.30Cr+0.37Ni+2.67Mo), which is a relational expression of the contents of Mn, Si, P, Cr, Ni, and Mo, satisfies 2.0 or more, cooling the hot-rolled steel sheet to a temperature of 500° C. to 650° C. at a cooling rate of 5° C./s or more, coiling the cooled steel sheet, holding the coiled steel sheet in this temperature range for 10 minutes or more, then cooling the held steel sheet to a temperature lower than 500° C. to obtain a hot-rolled steel sheet, and forming the obtained hot-rolled steel sheet into a pipe to obtain an electric resistance welded steel pipe. In addition, Japanese Unexamined Patent Application Publication No. 2006-299413 states that, by manufacturing a hot-rolled steel sheet by using the method described above, it is possible to obtain a hot-rolled steel sheet having a microstructure including bainitic ferrite as a main phase, 3 vol % or more of martensite, and 1 vol % or more of retained austenite as needed, and that by forming the obtained hot-rolled steel sheet into a pipe, it is possible to manufacture an electric resistance welded steel pipe having a low yield ratio of 85% or less, a low-temperature toughness corresponding to a ductile-brittle transition temperature of −50° C. or lower, and excellent plastic-deformation-absorbing capability.
In addition, Japanese Unexamined Patent Application Publication No. 2012-172256 proposes a technique including controlling the chemical composition of a hot-rolled steel sheet to be one containing, by mass %, C: 0.03% to 0.11%, Si: 0.01% to 0.50%, Mn: 1.0% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%, Nb: 0.01% to 0.10%, Ti: 0.001% to 0.05%, B: 0.0005% or less, one, two, or all of Cr: 0.01% to 1.0%, Mo: 0.01% to 0.5%, and Ni: 0.01% to 0.5%, and the balance being Fe and inevitable impurities, in which Mneq (Mneq (%)=Mn+0.26Si+1.30Cr+2.67Mo+0.8Ni), which is a relational expression of the contents of Mn, Si, Cr, Mo, and Ni, falls within a range of 2.0% to 4.0%, and controlling the microstructure of the hot-rolled steel sheet to be one including bainitic ferrite as a main phase, and at least 3.0%, in terms of area ratio, of martensite as a second phase, in which the average grain diameter of the bainitic ferrite is 10 μm or less. In addition, Japanese Unexamined Patent Application Publication No. 2012-172256 states that, by controlling the main phase of a hot-rolled steel sheet to be bainitic ferrite having an average grain diameter of 10 μm or less, it is possible to achieve a desired high strength after pipe making has been performed and to obtain a hot-rolled steel sheet having excellent low-temperature toughness. In addition, Japanese Unexamined Patent Application Publication No. 2012-172256 states that, by controlling the second phase to be a microstructure including 3.0% or more, in terms of area ratio, of martensite dispersed, it is possible to achieve a low yield ratio. Moreover, Japanese Unexamined Patent Application Publication No. 2012-172256 states that, by specifying the chemical composition and microstructure of a hot-rolled steel sheet as described above, it is possible to obtain a high-strength hot-rolled steel sheet with a low yield ratio excellent in terms of low-temperature toughness undergoing only a little decrease in strength after pipe making has been performed and having a yield strength in a direction at 30 degrees from the rolling direction of 480 MPa or more, a ductile-brittle transition temperature vTrs in a Charpy impact test of −80° C. or lower, and a low yield ratio of 85% or less.
However, in the conventional techniques described above, it is very difficult to obtain a hot-rolled steel sheet that can preferably be used as a material for an X80 grade linepipe. That is, it is very difficult to obtain a thick hot-rolled steel sheet having a high strength, excellent low-temperature toughness, a sufficient low-yield-ratio property which is effective against forced deformation caused by, for example, intense processing conditions when pipe making is performed or crustal movement after a pipeline has been constructed, and excellent stability of properties after processing has been performed (after pipe making has been performed).
In Japanese Unexamined Patent Application Publication No. 63-227715, there is a problem in that the hot-rolled steel sheet does not have strength as an X80 grade and in that there is a significant decrease in production efficiency because, for example, an air cooling process is included in a cooling process. In Japanese Unexamined Patent Application Publication No. 2006-299413, it is not possible to stably achieve a ductile-brittle transition temperature vTrs of −80° C. or lower, which is required for a cold-area-specification material having a good low-temperature toughness for which there is a growing demand nowadays. In addition, since steel having a comparatively good low-temperature toughness has a low strength, there may be a situation where the steel does not have strength as an X80 grade in, for example, a spiral steel pipe subjected to smaller forming strain than an electric resistance welded steel pipe.
In Japanese Unexamined Patent Application Publication No. 2012-172256, a decrease in strength after pipe making has been performed is suppressed by controlling a microstructure to be one including martensite and, optionally, bainite as a second phase. However, when only martensite or bainite is dispersed as a second phase, the degree of work hardening widely varies depending on the amount of forming strain when pipe making is performed. Therefore, for example, in an electric resistance welded steel pipe where there is usually a difference in the amount of forming strain between a position located at 90 degrees and a position located at 180 degrees (in the circumferential direction from the welded part which is assumed to be located at 0 degrees), there is a problem in that properties, in particular, a yield ratio varies with location in the circumferential direction of the pipe. When a yield ratio varies in the circumferential direction of the pipe as described above, there may be a problem in that the steel pipe undergoes buckling deformation because deformation is concentrated in a portion having a low yield ratio (low yield strength) when the steel pipe is deformed by being subjected to an external force caused by, for example, a crustal movement such as land subsidence or earthquake. Once a steel pipe undergoes buckling, since deformation is concentrated in the portion where buckling has occurred, the steel pipe tends to fracture because this portion further deforms.
It could therefore be helpful to provide a hot-rolled steel sheet which can preferably be used as a material for an X80 grade electric resistance welded steel pipe or a material for an X80 grade spiral steel pipe having a high strength, a high toughness, a low-yield-ratio property, and excellent stability of properties after pipe forming and to provide a method of manufacturing the steel sheet. Specifically, it could be helpful to provide a hot-rolled steel sheet having a tensile strength of 650 MPa or more, a yield strength of 555 MPa or more, a yield ratio of 90% or less, and a ductile-brittle transition temperature vTrs in a Charpy impact test of −80° C. or lower with which it is possible to control a variation in yield ratio ΔYR in the circumferential direction of the steel pipe (having a strain t/D×100 in the circumferential direction of the steel pipe of 1% or more and 9% or less, where D denotes the outer diameter of the steel pipe and t denotes the thickness of the hot-rolled steel sheet before pipe making is performed) of less than 10% after pipe making has been performed, and to provide a method of manufacturing the steel sheet.